Hi Dougie,
According to Hans in Steel boats, Iron Hearts the vierling was replace with the 3.7cm M42 after the 5th patrol 1st-13th July and before the 6th (aborted) patrol 1st-2nd August. Probably not that big of a job, unbolt one gun remove, refit with another, they would probably have been designed to have a common fitting bolt pattern, if not they had 2 weeks to make it fit, if there were ammunition racks inside the ready lockers then change them for the different munition. Page 145 in 2013 English paperback edition.
Speaking of said book, anyone want to comment on the picture on page 65 which appears to show the U505 with early Zscech tower emblems (battle axe on the side, possibly Olympic rings on the front), now for the interesting bit, 10.5cm deck gun, but not other armament?
regards
Jon